The invention relates to a screw for medical purposes having a screw body with a head portion and a following shaft portion having an outer threading. The screw is provided with a recess on which a tool can be applied for turning the screw. The invention further relates to a tool for driving such a screw.
Such screws are disclosed in OP-Journal 14 (1998,) pages 278-284 “Biodegradierbare Interferenzschrauben in der Kreuzbandchirurgie”, A. Weiler et al., Georg Thieme Verlag, Stuttgart, N.Y.
Interference screws have the purpose of anchoring a tendon or a ligament transplant to a bone. A channel is drilled into the bone in which the transplant is to be inserted. The screw is driven into the intermediate space between the transplant and an inner wall of the channel, so that the transplant is then clamped between the screw and the inner channel wall.
Considerable forces arise in such a tendon or a ligament, for example the cruciate ligaments in a knee joint, so that the clamping force must be correspondingly high to ensure a reliable anchoring. The screw is therefore provided with an outer threading which penetrates into the bone material at the inner side of the bore channel. At the same time, the outer threading engages with the transplant to be anchored.
Other fields of application for screws exist for the interconnection of materials. A central opening is provided in the head for rotating the screw, in which a tool can be placed, for example a corresponding screw driver. Since considerable forces are exerted on the screw, especially at the last few turns before the final seating, this opening is provided not only in the head region, for driving means engaging at the head, but extends centrally and axially far into the shaft region. The forces from the tool are then not only transferred to the head but are distributed over larger regions of the screw body. This is done especially to prevent the head from being sheared off from the rest of the screw body, especially for the last turns before the final seating.
It is known to produce such screws of metals, in particular titanium, however screws of biodegradable material have also found wider application. It has been found that biodegradable screws made of suitable materials have the same high initial anchoring strength as metal screws. However, considerable difficulties arise when driving the screws.
If the central opening has the form of a hexagon, into which a tool in the form of a hex head socket wrench is inserted, the danger exists, especially for biodegradable materials, that the hexagonal contour is destroyed and the tool then turns without frictional connection. For this reason, attempts have been made to select other cross-sectional forms of the central opening, which allow a greater surface contact with the tool. For example, FIG. 3 of the above-mentioned publication in the OP-Journal shows such cross-sections, in the form of a cross or a turbine. The tendency is to produce a relatively large and branched central opening, so that only a relatively thin wall remains for the screw body.
This has the disadvantage, especially for small and thin screws with very thin walls that the twist resistance is no longer sufficient to withstand the high forces applied when driving in the screw. This tendency of increasingly larger central openings also leads to the screw body having a relatively large central hollow volume. This has a disadvantage, especially for biodegradable screws that when the thin wall of the biodegradable body is biologically degraded after a relatively short time, a large hollow space results. With this, a sufficient clamping force is no longer present to hold the tendon or ligament transplant in place.
It is therefore an object of the present invention to overcome the above problems and to provide a screw which can reliably receive high driving forces even for smaller and thinner constructions, without being deformed or damaged.